SUMMARY
 It would seem that the transition to libinput, away from evdev, resulted in the additional mouse settings to go with it (eg. "Advanced" and "Keyboard Navigation" tabs).

STEPS TO REPRODUCE
 1. Go to "System Settings" -> "Input Devices" -> Mouse

OBSERVED RESULT
 The only settings are:
 General
 Point speed
 Acceleration profile
 Scrolling

EXPECTED RESULT
 "Keyboard Navigation" would be included.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch Linux
(available in About System)
KDE Plasma Version: 5.15.1
KDE Frameworks Version: 5.55.0
Qt Version: 5.12.1

ADDITIONAL INFORMATION
 References:
 https://gitlab.freedesktop.org/libinput/libinput/issues/249#note_127872 (first brought here in case the issue was with a missing libinput feature... It is not)
 https://userbase.kde.org/System_Settings/Input_Devices#Mouse
 https://forum.kde.org/viewtopic.php?f=289&t=153755#p405711
 https://bugs.kde.org/show_bug.cgi?id=398610 (unrelated? In case you were thinking of making closing this due to "DUPLICATE")